Components for use in aggressive, hot and corrosive conditions are mostly made of special high temperature alloys based on nickel or cobalt. These alloys are damaged during use, by high temperature corrosion. Due to the high temperatures and accelerated by the presence of even the smallest proportions of chlorine and sulfur, nickel and cobalt are converted to their oxides and sulfides. The oxides and sulfides of nickel and cobalt are, however, connected only loosely to the metallic undersurface, and scale off little by little.
It is known that nickel and cobalt alloys, beginning from a certain proportion of aluminum content, have an increased stability against high temperature corrosion. At high temperatures, these alloys develop on their surface a strongly adherent layer of aluminum oxide which regenerates by itself if the layer is damaged. In order to be able to select the base material according to strength criteria, methods for diffusion coating were developed, such as alitizing, by which the base alloy may be enriched retroactively with aluminum in the region near the surface.
An aluminization method is described in U.S. Pat. No. 3,102,044, in which a layer of aluminum is deposited on the component part. During the subsequent heat treatment, the aluminum melts and reacts with the substrate to form NiAl.
European Published Patent Application No. 0 748 394 describes an aluminide coating, enriched with platinum, that is modified with silicon. In order to prepare this layer, in a first step, platinum is deposited on the surface of a nickel superalloy. Subsequently, an aqueous suspension of pulverized elementary aluminum and silicon is applied and dried. Finally the aluminum is molten, whereupon the silicon dissolves and aluminum atoms and silicon atoms diffuse into the substrate.
Another protective layer against high temperature corrosion is made of MCrAlY (M=Ni, Co; CrAlY=chromium aluminum yttrium).
German Published Patent Application No. 35 35 548 describes a method for preparing a an MCrAlY coating. By galvanic dispersion deposition, a metal matrix is produced of Ni or Co having intercalated particles made of CrAlY. Upon subsequent tempering, the actual protective layer is then created.